One of the most commercially significant applications of electrolysis is the production of halogens, particularly chlorine, and alkali metal hydroxides, particularly sodium hydroxide, by the electrolysis of alkali metal halide solutions in diaphragm-type electrolytic cells. The configuration and operation of a diaphragm-type electrolytic cell is well known to those skilled in the art. A cell generally consists of three basic elements, that is an anode base, a cathode can, and a cover portion. The anode base serves as both anodic conductive member of the cell and as a support for the anode risers which extend in parallel between the parallel cathode tubes disposed in the cathode can. In one design of such a cell, anode plates extend vertically from a sealed cell base and a cathode can comprising four sides with a plurality of transverse, vertically-oriented, rectangular cathode tubes is positioned so that the anode plates are intermediate adjacent pairs of cathode tubes. A cover is then placed above the cathode can and the anodes which cover contains the required hyraulic head of brine solution and the collector for halogen gas produced at the anodes. The cathode tubes generally have a foraminous structure and, in addition to serving as the cathodic electrolysis surface, the external surfaces of the cathode tubes serve as a support structure for the diaphragm which is often a layer of asbestos fibers serving to separate the anode and cathode compartments of the cell.
As originally conceived, diaphragm cell anodes were formed of graphite. The problem of surface erosion due to factors such as gas formation thereon soon led to the development of the dimensionally stable anode comprising a valve metal base having an electrocatalytically active metal or metal oxide coating applied to the surface thereof. Such dimensionally stable anodes were originally conceived in box form and because of the variation in diaphragm thicknesses, they had to be of a somewhat narrow width so that they would not interfere with the diaphragm during installation of the cathode can onto the anode base after the anodes were in position. This necessity of narrow box anodes resulted in relatively wide anode to cathode spacings. However, since erosion of the anode was no longer a problem, these box-form anodes found wide acceptance as replacements for former graphite anodes in diaphragm-type electrolytic cells.
In recent years, the box-form anode has been replaced in new diaphragm cell installations by an expandable-type anode. The expandable anode is utilized in a compressed form during installation and once the cathode can is in position, spacer members may be either inserted or compression clips removed from the anode structure so that the anode surfaces may resiliently expand outwardly to close the gap between the anode and cathode surfaces resulting in significant savings in energy due to the lowered electrical resistance within the reduced gap. An anode of this type is described in U.S. Pat. No. 3,674,676.
While the economic advantages of expandable anodes are well known, it would be extremely expensive for a chlorine and caustic producer to dispose of his old style box anodes and replace them with the new, energy-saving expandable type. The fabrication cost, as well as the cost of materials for completely new anodes, would be prohibitive.
It is therefore a principal object of this invention to make use of the old style box anode as a basis structure which could be converted to allow chlorine and caustic producers now utilizing such box anodes to obtain the economic benefits of expandable type dimensionally stable anodes without the high capital cost of disposal of old box anodes and purchase of all-new expandable types.
It is a further object of this invention to utilize all or nearly all of the existing box anode structure in an economically feasible conversion to expandable anodes.